1. Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically, to a reflective liquid crystal display device which is used for OA (office automation) apparatuses such as word processors and notebook type personal computers, various image display apparatuses, and apparatuses for playing games.
2. Description of the Related Art
In recent years, the application of a liquid crystal display device (LCD) to a word processor, a lap-top personal computer, a portable television set called a "pocket TV", or the like has been rapidly developed. Conventionally, a twisted nematic (TN) mode and a super-twisted nematic (STN) mode are employed in such an LCD.
In the above-mentioned TN mode, a monochrome display (a black and white display) is conducted by using the optical characteristics of a liquid crystal display device (liquid crystal panel) interposed between a pair of polarizing plates, i.e. the optical rotatory characteristics when no voltage is applied and the optical non-rotatory characteristics when a voltage is applied.
In the STN mode, which is widely used for a display mode of a word processor, a twist angle of liquid crystal layer is set to be in the range of 180.degree. to 270.degree. in a liquid crystal panel of a similar structure to that in the TN mode. According to the STN mode, the twist angle of liquid crystal layer is increased by 90.degree. or more and the setting angle of the polarizing plates is optimized, thereby realizing an electro-optic characteristics with a steep threshold characteristic which reflect the rapid change of the orientation of liquid crystal molecules with an application of a voltage on the change of the birefringence of the liquid crystal layer. Since the STN mode has a steep threshold characteristic, the STN mode is suitable for simple matrix driving.
In the STN mode, the birefringence of the liquid crystal layer causes the background of the display to be colored in yellowish green or dark blue. In order to improve this drawback, an LCD is proposed in which a monochrome display is realized by means of color compensation which is conducted by attaching an optical compensation panel or a phase plate formed by a polymer such as polycarbonate to the liquid crystal panel of the STN mode. An LCD of such a structure, so-called "paper white LCD", is currently on the market. The detailed operational principle in the TN mode and STN mode is already well known in the art and is disclosed in "Liquid Crystal Device Handbook", Ed. by Japan Society for Promotion of Science, Committee 142, 1989, pp. 329-352.
In such an LCD of the TN or STN mode, however, if the LCD is used as a reflective LCD, the obtained display is undesirably darkened, because two polarizing plates interposing a liquid crystal panel are used. In order to solve the problem, reflective LCDs of the STN mode with a single polarizing plate are proposed in Japanese Laid-Open Patent Publication Nos. 4-97121 and 4-289818.
However, in the proposed LCDs mentioned above, none of the means to solve the following problems is mentioned: the prevention of the coloring of the displayed color during a gray-scale display, the retardation value of the liquid crystal layer, the retardation value of the optical compensation plate (phase plate), and the related position of the polarizing plate and the optical compensation plate. Consequently, in such LCDs, the prevention of the coloring of the displayed color cannot be realized during a gray-scale display.